Ancient Macedon was renowned for its military might. Phillip II introduced a new kind of infantry known as the Macedonian phalanx, in which each soldier carried a long spear (called a sarissa) that was approximately 13 to 20 feet long. The tight formation of the Macedonian phalanx formed a wall of spears, which was considered nearly impenetrable. In 334 B.C., Phillip II's son, Alexander led the Macedonian army across the narrow straights of the Hellespont into northwest Turkey. In one long military campaign that lasted 11 years, he conquered the Persian Empire, making Macedonia the largest, most powerful empire in the world.